Coastal Cities are always going to suck if all FXS does is try to make them "as good" as non-coast all cities. Coastal Cities have an inherent "negative" in Civ because they are a little more exposed militarily and in terms of loyalty. And as
@Victoria has pointed out, they get smashed by climate change much more than inland cities. Adding buffs here and there is also not transparent / gamey.
Coastal Cities need a real reason to be preferred that ideally emerges from the underlying mechanics. They need to be distinct versus inland cities.
Very, very broadly the current design of Civ VI seems to be inland cities are more inclined to have all the hammers and science and resources. Coastal Cities seem to be inclined to have all the gold. ( I'm not sure there's meant to be a huge different in how tall these cities are - coastals actually have more housing as default, both get the same housing from rivers, and both potentially have access to more food and housing via improvements although we could quibble over whether farms are better than fish.)
Thing is, Gold isn't a great use case for Coastals in the current meta. I think overall Coastals do have more gold potential because they can build both CHs and Harbours, and Sea Resources give gold. But you can get more than enough Gold from inland cities and from trade and cards and from trading with the AI and from Pillaging. Oh my!
Then there's chopping, which makes gold, hammers and food all largely irrelevant anyway, and chopping clearly favours inland over coastal cities. You can't build Space Ports with Crabs, only Stones and Jungle (although Crabs will buy you great people etc.).
As I said, I think the original design was for Coastals to have more trade routes by allowing trade stacking, which actually fit really well with the historical advantages of coastals and gave Coastal Cities a more destinctive advantage. I'm fine with no more stacking - too much micro - but it does mean Coastals don't have a big thing they are particularly good at early game which undermines their desirability versus inland Cities.
Funnily enough, FXS have sort of brought back some stacking and or trade focus given both Harbours and CHs get bonuses from City States now (I assume they stack) and the new trade efficiency rules. I just don't think these are enough of an incentive.
I really don't think moving Harbours in the Tech Tree is the solution, and it would actually undermine the current Coastal City bonus that they can build boats from the start. That said, I think maybe Commercial Hubs need to be harder to get or Trade Routes from markets need to be harder to get, to offset how it's more difficult to get Harbours up and return Coastals to being the "trade" Cities versus inland Cities being the "make units and science" Cities.
Basically, the game should incentivise players to beeline Harbours for trade, not Commercial Hubs. Currently it does the opposite.
Off the top of my head, I think maybe one or more of the following could help:
- Harbours give Trade Routes without the Lighthouse whereas Commercial Hubs need Markets. That would make Harbours way more powerful, although would also lose the current symmetry.
- Make Harbours cheaper than Commercial Hubs would maybe work (maybe via a policy card, a bit like Encampments), although that would make the RND and Cothon super cheap (less of an issue if the discount is tied to a policy card).
- Give Coastal Cities a free Trading Post, or awarding a free one once they build a Lighthouse would also help. This seems like a no-brainer to me, although maybe isn't all that powerful overall.
- Make Coastals more important to an empire's overall economy; eg after say Steel, Cities get +1 Hammer for being connected by rail to a City with a Shipyard or for having a Shipyard. Or just have some Policy Cards that better leverage Harbours etc. Or Harbours give Major Adjacencies to Commercial Hubs, IZ and Campuses (not just Commercial Hubs).
- Temper some of the current negatives for Coastal Cities (although this won't really help make them different). eg +1 Housing from fishing boats instead of +0.5 Housing (or maybe increasing to +1 Housing after Cartography or after you build a Lighthouse or Shipyqrd); or maybe Sea Ports provide some protection from Storms and or Flooding.
Circling back to the OP, I really like the idea of Coastal Flooding and Climate Change. But if we're going to have that, there needs to be more "reward" for having coastal cities to offset the additional "risks" involved. And overall, Coastal Cities need more of a reason to be in the game beyond just more gold.